The present invention relates to label distribution in a label switched network and, more particularly, to resolving contention in networks that permit unidirectional and bidirectional label switched paths.
Multi-Protocol Label Switching (MPLS) is an advanced framework for high-speed data forwarding that differs from conventional destination-based IP routing: each packet is provided with a “label” that is used by label switched routers (LSRs) to forward the packet along what is referred to as a label switched path (LSP). See E. Rosen et al. “Multiprotocol Label Switching Architecture,” Internet Engineering Task Force (IETF) Network Working Group, Request for Comments (RFC) 3031. LSRs inform adjacent nodes of label bindings using a process of label distribution which is known as LSP setup. See. e.g., L. Andersson et al. “LDP Specification,” IETF Network Working Group, RFC 3036. Labels are by convention allocated and distributed from a downstream direction—where “downstream” in the art refers to the direction of data flow. In the context of IP networks, where LSPs are assumed to be unidirectional, downstream label selection convention assures that there is no label contention among connection requests coming from different directions.
Generalized MPLS (“GMPLS”), also referred to in the art as Multi-Protocol Lambda Switching (“MPL(ambda)S”), extends MPLS to support—not just packet-switching devices—but devices that perform switching in the time, wavelength, and space domains. GMPLS provides the potential for a control plane that can be utilized with legacy equipment (e.g. SONET) as well as newer devices (e.g. optical crossconnects “OXCs”)). See, e.g., D. Awduche et al. “Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering Control with Optical Crossconnects,” IETF Network Working Group, Internet Draft. For various practical reasons, current GMPLS signaling mechanisms permit the setup of what are referred to in the art as bidirectional LSPs. See P. Ashwood-Smith, et al., “Generalized MPLS—Signaling Functional Description,” IETF Network Working Group, Internet Draft. The introduction of bidirectional LSPs creates the practice of upstream label distribution and suggested label distribution. These two label distribution policies present the possibility of contention between two bidirectional LSP requests traveling in opposite directions between two adjacent nodes. If there is no restriction on the ports/channels that can be used for bidirectional LSPs and if there are alternate resources, then both nodes will pass different labels upstream/downstream and the contention will be resolved naturally. If there is a restriction on the ports/channels that can be used for the bidirectional LSPs (for example if they must be physically coupled on a single I/O card), or if there are no more resources available, then the contention must be resolved by some other means. The current GMPLS signaling proposal suggests letting the node with the higher node ID win the contention.
Unfortunately, current GMPLS proposals do not address the situation when a unidirectional LSP and a bidirectional LSP compete for the same resources. A network operator, in fact, may wish to offer both unidirectional and bidirectional high speed connections over the same network—without incurring the possible provisioning costs of segregating interfaces on network nodes between such connections. In a shared unidirectional and bidirectional LSP GMPLS network, a unidirectional LSP setup request message may carry a suggested label; the corresponding reply message may carry a label based on downstream label distribution policy. A bidirectional LSP setup request message should carry an upstream label and may carry a suggested label; the related reply message may carry a label based on downstream label distribution policy. The above-mentioned prior art contention schemes for handling bidirectional and unidirectional LSPs, however, are not consistent. Accordingly, there is a need for enhanced contention resolution procedures that can handle such situations.